Secondary batteries have been used in each vehicles such as automobile, aircraft, and agricultural machinery, as well as the various pieces of equipment such as digital cameras, notebook computers and mobile phones. Among them, in recent years, sodium sulfur battery has been attracting attention as a secondary battery that can store large amount of power.
Sodium-sulfur battery has a positive electrode chamber where molten sulfur which is positive electrode active material is housed, negative electrode chamber where molten sodium which is negative electrode active material is housed, and wall-shaped solid electrolyte which is made of materials such as β alumina having a permeable to sodium ions, is located between said positive electrode chamber and said negative electrode chamber, and isolate the molten sodium and the molten sulfur.
In this sodium-sulfur battery, in a state of being heated to operating temperature of 290-350° C., molten sodium in the negative electrode chamber transmits through the solid electrolyte, acts as a sodium ion, reacts with sulfur in the positive electrode chamber, then discharge is carried out to generate a sodium polysulfide. Also, when charging, the reverse reaction proceeds at the time of discharge, sodium polysulfide is decomposed, sodium and sulfur are generated.
In this case, molten sulfur which is positive electrode active material, and sodium polysulfide which is reaction product have large specific gravity. Especially the sodium sulfide has large specific gravity because it is high ratio of sodium. For this reason, they tended to accumulate at the bottom of the positive electrode chamber by its own weight. Thus, these substances are not able to involved fully in charge and in discharge, the utilization of active material has decreased.
In order to prevent such a phenomenon, in Japanese Patent Laid-Open No. H5-266921(1993), a shape of the positive electrode chamber of sodium-sulfur battery, formed on the outer peripheral side of the bulkhead β-alumina tube that houses the molten sulfur as a positive electrode active material has been disclosed. The shape of the positive electrode chamber is formed such that the horizontal cross-sectional area becomes smaller at the bottom.
As a result, sodium ratio of sodium polysulfide of the bottom of the positive electrode chamber is lower than the ratio of the top at the time of charging, and this prevents the sodium sulfide which is a high percentage of sodium from concentrating at the bottom of the positive electrode chamber.
In addition, in Japanese Patent Laid-Open No. H6-89739(1994), a sodium-sulfur battery which is formed that the bulk density of the lower part of the conductive material for the positive electrode impregnated with molten sulfur is higher than the bulk density of the upper part is disclosed. Thus, reactions of the active material like sodium polysulfide occur rapidly, utilization at the lower part of the active material is improved.
And it says that the phenomenon that sodium polysulfide is accumulated at the lower part can be prevented.
Further, sodium-sulfur battery has been receiving attention as energy storage, because it has an ability to store large amounts of power. As a method to increase the battery capacity of the sodium-sulfur battery, Japanese Patent Laid-Open No. 2004-178991 can be mentioned as an example.
In this case, a way of to increase the battery capacity by increasing the packing densities of molten sulfur which is the positive electrode active material and molten sodium which is the negative electrode active material has been disclosed.
However, by the above method, there is a limit to increasing the battery capacity per unit cell because capacities of the positive electrode chamber where molten sulfur which is positive electrode active material is housed and the negative electrode chamber where molten sodium which is negative electrode active material is housed are constant.
On the other hand, if the capacity of the storage chamber where active material is housed is increased, it is also possible to store large amounts of active material. However, increasing the capacity of the storage chamber causes the cell reaction be reduced because area which acts as a partition wall of the solid electrolyte in contact with the active material becomes relatively small. In addition, since storage chamber is a space formed by being partitioned by the solid electrolyte that stores large amount of active material in there, there is a problem from the viewpoint of safety.
In addition, in Japanese Patent Laid-Open No. S50-38030(1975), a sodium-sulfur battery is disclosed. In the battery, molten sodium is housed in a sodium storage container which is located separately and remotely from a solid electrolyte. And sodium is supplied to pores provided in the plate-shape solid electrolyte from this storage container.